Electrolyte and process for the electrodeposition of cadmium



United States Patent 3,442,776 ELECTROLYTE AND PROCESS FOR THE ELECTRODEPOSITION 0F CADMIUM Wolfram Ruff, Frankfurt am Main, and Heinz Boucher, Neu Isenburg, Germany, assignors to Alfred Teves KG, Frankfurt am Main, Germany, a corporation of Germany No Drawing. Filed Sept. 19, 1966, Ser. No. 580,203 Claims priority, application Germany, Sept. 25, 1965,

T Int. Cl. C23b 5/10 US. Cl. 20450 Claims Our present invention relates to a process for the electrodeposition of cadmium layers or coatings upon iron and steel substrates and, especially, to the galvanic coating of spring-steel members with carmium from a cyanidecontaining electrolyte.

In the technology of cadmium electroplating, the electrolytes have hitherto been primarily highly alkaline solutions which contain cadmium ion and cyanide ion in a relatively high cyanide cadmium ion ratio. Caustic soda or sodium hydroxide is added to such a bath in a relatively high proportion by comparison with the metal (i.e., cadmium) content thereof in order to improve the nature of the cadmium deposition, its coating capabilities and throwing power. Such proportions also avoid disadvantages inherent in the use of high cadmium-ion concentrations, namely the tendency of the deposit to have a matte or dull finish. Electroplating is carried out, with such electrolytes, by either the barrel-plating or the still-(rack) plating systems against high-purity cadmium anodes.

The major disadvantage of conventional cadmium electroplating techniques using cyanide-containing electrolytes derives from the tendency of hydrogen to be electrolytically generated at the cathode. The atomic or molecular hydrogen produced at the cathode, i.e., the electrodeposition surface, tends to dilfuse into the metal layer (i.e., the cadmium coating or the substrate) and to embrittle the surface zones of the metallic body. As a consequence, pitting of the cadmium layer or the substrate may occur. Cathodic generation of hydrogen increases with decreas ing cadmium-ion concentration in the electrolyte.

It is, therefore, the principal object of the present invention to provide an improved method of electroplating cadmium from a cyanide-containing electrolyte which deposits the cadmium with excellent coverage upon iron or steel substrates (e.g., spring steel) and is free from the disadvantages characterizing earlier cadmium-plating methods.

A further object of this invention is to provide an electroplating bath for the cadmium coating of iron and steel bodies which eifectively limits the generation of hydrogen at the cathode and thereby improves the quality of the electrode plate.

Still another object of our invention is to provide a method of electroplating cadmium onto iron or steel bodies with high throwing power, excellent electrodeposition coverage, and the formation of a high-strength, tenacious and relatively uniform coating.

We have discovered, surprisingly, that these objects can be obtained and the electrolytic generation of hydrogen avoided, with production of a hard bright layer, when the electrolyte is an aqueous solution free from any addition of an alkali hydroxide and containing cyanide ions and cadmium ions in a ratio of 1.0 to 1.2 and having a cadmium-ion concentration of 45 to 55 g./liter; these limits are critical for the purposes of the present invention. While the bath may contain other additives which have proved to be effective for the electrodeposition of bright cadmium deposits from cyanide-containing electrolytes, we have found that best results are obtained when the electrolyte is free from any of the usual brighteners and any other additives so that it can be said to consist essentially of 45 to 55 g./liter of cadmium ion, sufiicient cyanide ions to provide a cyanide/cadmium ion ratio of 1.0 to 1.2, and associated ions (corresponding to cadmium oxide and alkali-metal cyanide) in aqueous solution. With an electrolyte of this character, it has been found that a high-rate electrodeposition of cadmium can be carried out with good throwing power and surface uniformity upon iron and steel bodies, especially spring steel, without any generation of hydrogen at the cathode so that the disadvantages inherent in hydrogen contamination of the workpiece and its surface zones can be completely avoided.

Our investigations have shown that this advantage is derived in large measure from the fact that, in the preparation of the electrolyte by dissolving in aqueous medium a cyanide-containing compound and a cadmium-containing compound to obtain the ratios and the concentration stated, no alkali hydroxide is added. It has also been observed that these advantageous results can also be obtained if, in addition to avoiding the introduction of alkali-metal hydroxide, the cadmium-ion concentration is maintained strictly between the narrow limits of 45 to 55 g./ liter and the cyanide/cadmium ion ratio is maintained between the limits of 1.0 and 1.2.

It has been found further that the elimination of any tendency to generation of hydrogen permits workpieces of complicated shape and intricate configuration to be electroplated with cadmium in a uniform manner; the tendency of such complicated shapes to entrap gases which block the electroplating operating is eliminated, and it is unnecessary to provide suction means at the edges of the bath for drawing off the nascent gases. After handling of the workpieces is no longer complicated by possible embrittlement or spalling of the electrodepo'sited layer. Moreover, the quality of the bath does not deteriorate as rapidly so that the electrolyte generally has a much longer usable life, lower replenishment cost and improved effectiveness for a longer period than is the case with conventional caustic-soda cyanide baths.

According to another feature of this invention, electrodeposition is carried out with relatively high current density (up to 5 amps/dm with the bath at about room temperature (i.e., 20 to 25 C.) and, in any event, not

greater than 50 C. Under such conditions, the electrodeposited layer is found to be relatively hard, ductile and tough, with little tendency to scale or peel from the substrate. Preferably a current density between 2.5 and 4 amps/dm. is employed at room temperature;v The high current density permits a substantial shortening of the electroplating time, thereby effecting still a further reduction of the cost of the process, which cost is quite low as a consequence of the omission of brighteners as noted above. Another feature of this invention is that it appears to have eliminated the disadvantages of conventional systems at high current densities and interelectrode voltages; thus conventional systems under these conditions are characterized by a polarization which results in the formation of cadmium oxide at the anode and a consequent decomposition thereof to form oxygen. The development of oxygen and polarization effects are completely obviated by using the bath of the present invention.

EXAMPLE A cadmium-plating bath is prepared by dissolving 110 g. of 95% liter sodium cyanide in one liter of water; after heating the solution at room temperature (i.e. 20 to 25 0.), 60 g. of technically pure cadmium oxide (containing cadmium metal) is added to the solution and dissolved therein. The solution is cooled to room temperature and is free from any brightener, no alkalinemetal oxide having been added. Analysis shows that the electrolyte contains 50 g. of cadmium and 55 g. of cyanide per liter.

Electroplating is carried out with an interelectrode potential of 0.9 volt using cadmium anodes of 99.98% purity with a current density of 3 amps/dmf"; the cathode was a band of spring steel. In a ten-minute period, the band, which had a thickness of 0.5 mm., was provided with a coating of 12 microns (10l g./m. on each side, of bright uniform cadmium. Using conventional hardness testing methods, a ball of mm. diameter was forced into the workpiece with an average pressure load of 260 kp. A corresponding band of nonplated steel required a load of 250 kp. for the corresponding depression. Spalling and flaking of the cadmium layer was not observed even under such stress.

We claim:

1. A method of coating an iron or steel workpiece with cadmium, comprising the steps of:

(a) preparing an aqueous electrolyte having a cadmium-ion concentration of 45 to 55 g./liter and a cyanide/cadmium ion ratio of 1.0 to 1.2, by dissolving at least one cadmium-containing compound and at least one cyanide-containing compound in water without the addition thereto of an alkalimetal hydroxide; and

(b) electroplating metallic cadmium from the bath produced in step (a) upon said workpiece constituted as a cathode against a cadmium anode.

2. The method defined in claim 1 wherein said workpiece is electroplated with cadmium in step (b) at a current density of up to 5 amps/dm. at a temperature of said bath up to 50 C.

3. The method defined in claim 2 wherein said workpiece is electroplated with cadmium in step (b) at a current density of 2.5 to 4 amps/dmF, said bath being substantially at room temperature.

4. The method defined in claim 2 wherein said bath ,is free from cadmium-plating brighteners.

6. The method defined in claim 5 wherein said cadmium-containing compound is cadmium oxide and said cyanide-containing compound is sodium cyanide.

7. A method of making an electrolyte for the electrodeposition of cadmium upon an iron or steel workpiece, comprising the step of dissolving at least one cadmiumcontaining compound and at least one cyanide-containing compound in water to produce an aqueous solution having a cadmium-ion concentration of to g./ liter and a cyanide/cadmium ion ratio of 1.0 to 1.2, said solution being free from any addition of alkali-metal hydroxide.

8. The method defined in claim 7 wherein said cyanide-containing compound is sodium cyanide and said cadmium-containing compound is cadmium oxide.

9. An aqueous electrolyte for electrodepositing cadmium onto an iron or steel workpiece comprising a cadmium-containing compound in an amount sufficient to give 45-55 g./l. of cadmium ion in solution and a cyanide-containing compound in an amount such that the ratio of cyanide ion/cadmium ion is 1.0 to 1.2, said electrolyte being free from an addition of alkali-metal hydroxide.

10. The electrolyte of claim 9 wherein said cyanidecontaining compound is sodium cyanide and said cadmium-containing compound is cadmium oxide.

References Cited UNITED STATES PATENTS 2,143,760 1/1939 Camel 20450 2,892,761 6/ 1959 Hamilton et a1. 204-50 3,014,852 12/1961 Hamilton et al. 204-50 OTHER REFERENCES Cash, D. 1., et al: Metal Progress, 75, (February 1959).

Hamilton, W. F., et a1: Proceedings of the American Electroplaters Society, 47, (1960).

JOHN H. MACK, Primary Examiner.

G. L. KAPLAN, Assistant Examiner. 

1. A METHOD OF COATING AN IRON OR STEEL WORKPIECE WITH CADMIUM, COMPRISING THE STEPS OF: (A) PREPARING AN AQUEOUS ELECTROLYTE HAVING A CADMIUM-ION CONCENTRATIN OF 45 TO 55 G./LITER AND A CYANIDE/CADMIUM ION RATIO OF 1.0 TO 1.2, BY DISSOLVING AT LEAST ONE CADMIUM-CONTAINING COMPOUND AND AT LEAST ONE CYANIDE-CONTAINING COMPOUND IN WATER WITHOUT THE ADDITION THERETO OF AN ALKALIMETAL HYDROXIDE; AND (B) ELECTROPLATING METALLIC CADMIUM FROM THE BATH PRODUCED IN STEP (A) UPON SAID WORKPIECE CONSTITUTED AS A CATHODE AGAINST A CADMIUM ANODE. 